1. Field of the Invention
The present invention relates generally to the manufacture of plastic containers, and more particularly to the use of substantially flat surfaces on pinch bars to separate parisons used in forming containers manufactured in a dual parison blow-molding machine.
2. Related Art
Cut-off knife systems are currently used to cut extruded polymer tubes, or parisons, during the manufacture of plastic bottles. The cut-off knife systems use knives mounted to blow molds to cut the parison in the same fashion as scissors.
During the production of plastic bottles, for example high density polyethylene (HDPE) bottles, on a high-speed rotary wheel machine, problems are encountered when cutting the parison using the cut-off knife systems. If and when the cut-off knives are not perfectly spaced or sharp, the separation of the parison between two adjacent molds is not complete. Additionally, a dull knife or nicked knife can result in an incomplete cut and form a stringer, which forms a membrane-like connection between the molds. Stringers can cause problems when the bottles drop from the molds. For example, if the stringer does not break when the bottle drops, the bottles get thrown off balance and can fall onto the cleated conveyor, out of location and cause jam-ups downstream.
Further, in cut-off knife systems, the knives are mounted to the exterior portion of the mold that is located between adjacent molds. This arrangement results in large gaps between adjacent molds. When there are large gaps between adjacent molds, there is more heated parison present between the adjacent molds, which can also result in stringers.
In dual parison blow-molding, the problem of stringers is exacerbated because of the weight of the bottles. Bottles produced from dual parison systems tend to be light and the stringers are more likely to cause the bottles to fall out of location because the stringers can cock the bottle as it falls onto the cleated conveyor.
What is needed then is an improved method and apparatus for pinching off parison to separate parisons from adjacent molds to reduce the likelihood that stringers will occur.